<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>
    Array.prototype.myReduce = function (fn, initial) {
      let arr = this
      initial = initial ? initial : arr[0]

      let startIndex = initial ? 1 : 0

      for (let i = startIndex; i < arr.length; i++) {
        initial = fn(initial, arr[i], i, arr)
      }
      return initial
    }

    let res = [1,2,3].myReduce((pre, item) => {
      return pre + item
    })

    console.log(res);
  </script>
</body>

</html>